In cases like these I draw the line and oh how I cross it. I remember by friend from freshman year Anime Shaun. (the title being his grandmastery of all things animated and eastern

) Well, Shaun began seeing this mentally unstable girl by the name of Lisa, an emo-chick clone with raven black Beetles hair and more self-inflicted drama than Oedipus in a Mother-Son needlepoint competition. Not only did she insist on taking up all of his free time, but she followed him to all of his classes, and would break into frequent fights every time he slept in and didn't keep her company during hers. She disallowed him from hanging out with us for all but saturday night, D&D night... then when she caught him drawing his character portrait one time in his notebook, she ripped it up in a fit of jealousy... as if the little animate grey elf were actually a threat of some sort. Then she'd get all whiney and overbearing and keep yelling at me for being single and not hitting on every random girl I met, and started yelling at ME one time when I refused to go out with her roommate. I know... REALLY doesn't make any sense at all, right?
Then she crossed the line and started scheduling panic attacks for every D&D session... showed up in the dorm commons-room 3 consecutive weeks in a row crying over some stupid drama and yanking him out. After the 3rd time I talked to Shaun about it the next day about how I didn't like how insecure she was acting, that he needs breathing room or the whole relationship's going downhill... He was interrogated that night by her apparantly, and she immediately stormed up to my dormroom hitting me and cussing me out... by this point I just stood there laughing at her, being 6'6" and neigh indestructable... she paused confused and glared at me...
"Look, I don't care what happened in your past, whether your daddy beat you, God was out to get you or whatever you have in your pretty little swirling head right now... I don't hate you. Truth is I pity you. That somebody could walk around day after day in such a state and still not believe they need some ************ therapy!"
She punched me in the face and ran off, Shaun on her leash obediently following. The next day he told me that she was in the corner of his room in the fetal position crying for 6 hours with the sheets up over her head and screaming... and that I should apologize to HER. "Shaun," I said, "If that right there doesn't PROVE to you once and for all that she's too unstable for a relationship, I'm going to have to let you go man. There's a difference between being whipped and taking advantage of somebody who's not even legally mentally competent." He didn't seem mad or anything, kind of smirked actually, said ok and that was that, we didn't talk to eachother for about a year.
One year later, after a failed engagement when she cheated on him, KEPT his ring, tried to get her exes and brother to beat him up for various her lies, several thousand dollars of credit card bills, a decimated renter's lease, his pet rat MURDERED, and all sorts of physical abuse she sent his way, they finally split. The very DAY they broke up, I got the call on the cell..
"So John, you still running D&D saturday nights?"
Trust me man, stand your ground. She has a restraining order on you? Stay away from both of them and cut him off as well. The very fact he's still dating a woman that would hit you shows where he stands... and now they're married. If he truly is your friend, you'll hear from him in about a year when he's finalizing the divorce settlements and wants to go out on the town womanizing like old times again.